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CARSON WATER SUBCONSERVANCY DISTRICT
ALLUVIAL FAN MAPPING STUDY
Elise Jarrett
Water Resources Planner
U.S. Army Corps of Engineers
OVERVIEW
• Study History (Phase 1 & Phase 2)
• Geologic Assessment
• Hydraulic Assessment
• What’s Next?
STUDY HISTORY
• Phase One completed in 2017
o Mapped and classified 297 alluvial fans
based on apparent risk within the Carson
River Watershed
• Phase Two identified specific alluvial fans for
further geologic and hydraulic analyses
PHASE 1 RESULTS: DISTRIBUTION OF RISK RANKING
• A majority of the fans were
classified as moderate risk
• No fans were classified as very high
risk or as very low risk
• The Town of Dayton-Carson River
watershed had the most high risk
fans at 20
• West Fork Carson River watershed
had the highest proportion of high
risk fans at 76%
Very
LowLow Moderate High
Very
HighTotal
Number
of Fans0 18 202 77 0 297
Percent 0% 6% 68% 26% 0% 100%
GOALS FOR PHASE 2
• GOALS
1. Develop additional in-depth datasets for planners and city/county managers to
increase knowledge of alluvial fan risk.
2. Increase the certainty of alluvial fan delineations on selected fans.
3. Conduct hydraulic analysis on selected fans.
• SIGNIFICANCE
o To better understand and prioritize watershed risk, which would prevent further
development in high hazard areas and develop strategies to mitigate in areas
where development already occurs.
GEOLOGIC ASSESSMENT
• Goal
o Evaluate geohazards (areas
susceptible to floods/debris flows)
• Methodology
o 8 fans selected by County
Douglas County: (6 fans) 44, 45, 78,
79, 81 & 82
Carson City County: (2 fans) 115 &
116
• Fans had to have LiDAR data available
o LiDAR data downloaded from USGS
o 3DEP Elevation Data (https://viewer.nationalmap.gov/basic/)
GEOLOGIC ASSESSMENT
• LiDAR Assessment:
o Only use bare earth data from LiDAR
collection
o Digital elevation models
o Generated contours (1m, 2m, & 5m)
o Slope maps
Surface morphology
Calculate gradients
Define outer toe (distal edge) : 1° - 2°
Digital elevation model LiDAR Derived Contours (5 m)
Slope Model Examples
Uniform gradient for slope angles 0 -50° Slope Model 5 (0 -50°)
GEOLOGIC ASSESSMENT
• LiDAR Assessment (cont.):
o Fan boundaries
Contours superimposed on
LiDAR hillshade
Slope maps
Aerial photography
o Site visits
o Evaluate Geohazards
LiDAR Derived Contours (5 m) on
hillshade map
Sandy alluvial deposits of Fan 78
Light coloration inferred as zones
where recent deposition has occurred
Relative Risk
GEOLOGIC ASSESSMENT
November 2018
(Do not cite/reproduce)
GEOLOGIC ASSESSMENT
• Summary:
o All fans are active (varying degrees)
#79 & 82 pretty benign
#44 & 45 very active (steepest of fans)
o Alluvial transport & deposition occurring
Channel & sheet flow deposits common (active
indicator)
Fresh deposits along road during site visit
o Debris flow possible
#44 & 45 most susceptible
Steep sections near fan apex (#81 & 116; maybe
#78)
HYDRAULIC ASSESSMENTRain cloud image source: https://ya-webdesign.com/image/raining-clipart-rainy-sky/1678401.html
HYDRAULIC ASSESSMENTInundation Maps
Site 81
Site 82
Site 79Site 78
HYDRAULIC ASSESSMENT
• Maximum inundation depth at every location
• Inundation depths assuming no channel
movement for a 100-year event
HEC-RAS 2D Hydraulic Model
HYDRAULIC ASSESSMENT
• Assumptions
o Highly active alluvial fans
o Flow paths are allowed to move randomly
o Simple relationship between depth and flow
o Not valid for debris flow
FEMA FAN Program
• Only performed on highly active fans
• Looking to see what would happen if the channel moves – areas that could be
flooded
• Program uses peak flows and probabilities, rather than a 100-yr hydrograph.
• Program is mapping the area that has a 1% chance of being inundated, while
the HEC-RAS model is mapping the inundated depth during a 1% probability
event
3.5 ft/s
1/100 probability
of depth > 0.5 ft
NOT the inundated
area during a 1/100
event
HYDRAULIC ASSESSMENT
Inundation Map
3.5 ft/s
depth > 0.5 ft
+ =
FEMA FAN 2D HEC-RAS
Geologic Assessment
HYDRAULIC ASSESSMENT
Inundation MapsSite 115
HYDRAULIC ASSESSMENT
Inundation Maps
Site 44 Site 45
Warning: Debris or mud
flows are possible on
these alluvial fans, which
could result in much
deeper flows than
represented on these
maps.
WHAT’S NEXT?
• Understanding impacts to existing and future development
• Prioritizing watershed risk and develop strategies to minimize
that risk
• Utilizing identified alluvial fan areas and
associated risk in development review
Source: esri.com
QUESTIONS?
Elise Jarrett: [email protected], (916) 557-6622
Lew Hunter (Geology): [email protected], (916) 557- 5368
John Newton (Hydraulics): [email protected], (509) 527-7289
